crypto/x509: don't read symlinked root certs from disk twice

On Linux distros at least, it's common for cert directories to have
symlinks pointing to other certs or even other symlinks. An example from
Debian stretch's /etc/ssl/certs directory:

...
lrwxrwxrwx 1 root root     46 Aug 13  2018 106f3e4d.0 -> Entrust_Root_Certification_Authority_-_EC1.pem
lrwxrwxrwx 1 root root     49 Aug 13  2018 116bf586.0 -> GeoTrust_Primary_Certification_Authority_-_G2.pem
lrwxrwxrwx 1 root root     35 Aug 13  2018 128805a3.0 -> EE_Certification_Centre_Root_CA.pem
lrwxrwxrwx 1 root root     26 Aug 13  2018 157753a5.0 -> AddTrust_External_Root.pem
lrwxrwxrwx 1 root root     59 Aug 13  2018 1636090b.0 -> Hellenic_Academic_and_Research_Institutions_RootCA_2011.pem
lrwxrwxrwx 1 root root     23 Aug 13  2018 18856ac4.0 -> SecureSign_RootCA11.pem
lrwxrwxrwx 1 root root     31 Aug 13  2018 1d3472b9.0 -> GlobalSign_ECC_Root_CA_-_R5.pem
lrwxrwxrwx 1 root root     37 Aug 13  2018 1e08bfd1.0 -> IdenTrust_Public_Sector_Root_CA_1.pem
lrwxrwxrwx 1 root root     35 Nov  8 21:13 773e07ad.0 -> OISTE_WISeKey_Global_Root_GC_CA.pem
-rw-r--r-- 1 root root 200061 Nov  8 21:24 ca-certificates.crt
lrwxrwxrwx 1 root root     27 Nov  8 21:13 dc4d6a89.0 -> GlobalSign_Root_CA_-_R6.pem
lrwxrwxrwx 1 root root     62 Nov  8 21:13 GlobalSign_Root_CA_-_R6.pem -> /usr/share/ca-certificates/mozilla/GlobalSign_Root_CA_-_R6.crt
drwxr-xr-x 2 root root   4096 Jan 26  2019 java
lrwxrwxrwx 1 root root     70 Nov  8 21:13 OISTE_WISeKey_Global_Root_GC_CA.pem -> /usr/share/ca-certificates/mozilla/OISTE_WISeKey_Global_Root_GC_CA.crt
...

The root_unix.go code read those certs with same-directory twice before.

This drops the number of files read from 258 to 130. Saves about 20 ms.

Change-Id: I36a1b1e8bb8d89ed3dac8b6255f9048cb7f08fe8
Reviewed-on: https://go-review.googlesource.com/c/go/+/229918


Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Filippo Valsorda <filippo@golang.org>